Method for manufacturing a drilling tool with an elastomer seal having graded properties

ABSTRACT

A method of forming a seal for a subterranean drilling tool employs irradiation. The seal is formed of a single homogeneous elastomeric material. The seal has an exterior surface with at least one portion that is in sliding engagement with part of a seal gland of the drilling tool. A property gradient is formed in the elastomeric material on at least this portion. The property gradient is formed by exposing the seal to an environment to define a property that changes from a first level to a second level without essentially changing the composition of the elastomeric material. The step of exposing is done by irradiating the seal with an electron beam.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to provisional application 60/586,785filed Jul. 9, 2004.

FIELD OF THE INVENTION

This invention relates in general to manufacturing a seal for anearth-boring bit that has a property gradient portion formed on at leasta portion of the seal.

BACKGROUND OF THE INVENTION

One type of earth boring bit has a bit body with three rotatable cones.Each cone has cutting implements for disintegrating earth formations.The cones are mounted on bearing pins that depend from the bit body. Alubricant cavity within the bit body supplies lubricant to the bearingspaces between the cones and bearing pins. A seal or seal assembly islocated at the mouth of the cone to prevent loss of the lubricant.

One type of seal is an elastomeric annular member having an innerdiameter and an outer diameter. The cross-sectional configuration isrounded, such as circular or elliptical. One of the diameter portionsforms a dynamic seal. The dynamic seal portion is in sliding engagementand normally the other side of diameter portion forms a static seal.Sometimes both diameter portions encounter dynamic sealing engagement.The sliding engagement generates heat due to friction, thus causing wearover time.

Some drill bit seals are formed of a single homogeneous elastomericmaterial. The hardness of the elastomeric material is uniformthroughout, including at both the inner and outer diameter portions.Another type of drill bit seal is formed of two or more different typesof elastomer that are co-cured. One type has a greater hardness than theother type for serving as the dynamic seal portion. The softer portionprovides the desired amount of force due to the squeeze on the seal wheninstalled. It has also been proposed to plasma treat the surface of theseal with an inert gas containing a reactive gas species such aschlorine or fluorine. This treatment provides a chlorinated orfluorinated molecules at the surface. Various other techniques to changethe surface of an earth boring bit seal are disclosed in the patentedart.

SUMMARY OF THE INVENTION

In the method of this invention, a property gradient is formed in atleast a portion of the elastomeric material. Preferably, the propertygradient is formed by exposing the seal to an environment to creategradual change in at least one property of the elastomeric material,such as hardness, elastic modulus or bulk modulus. In one embodiment, anelectron beam is used to irradiate the seal, increasing cross-linking ofthe polymer chains within the property gradient. The elastomericmaterial within the property gradient is essentially chemically the sameas the remaining portion of the seal. Preferably, one property comprisesa decreasing hardness in the gradient from the exterior surface for aselected depth. A portion of the exterior surface may be free of anyproperty gradient.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of an earth-boring bit having a sealconstructed in accordance with this invention.

FIG. 2 is an enlarged sectional view of the seal of FIG. 1, shownremoved from the bit.

FIG. 3 is a further enlarged sectional view of a portion of the seal ofFIG. 1, shown installed within the bit.

FIG. 4 is a graph illustrating for the seal of FIG. 2 the hardnessversus the depth from the dynamic surface of the seal.

FIG. 5 is a sectional view of an alternate embodiment of a seal for anearth-boring bit of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, drill bit 11 has a bit body with at least one bitleg 13, and typically three legs. Each bit leg 13 has a dependingbearing pin 15. A cone 17 rotatably mounts over each bearing pin 15. Aseal 19 seals lubricant within a cavity of cone 17 surrounding bearingpin 15. A compensator 21 is in communication with the lubricant forequalizing the pressure of the lubricant with the drilling fluidhydrostatic pressure on the exterior of drill bit 11. Drill bit 11 couldalso be a type utilized with compressed air as the drilling fluid.

Referring to FIG. 2, seal 19 in this embodiment is an O-ring having acircular transverse cross-section. Seal 19 has an inner diameter 23 thatengages bearing pin 15 in a dynamic sliding contact as cone 17 rotates.Seal 19 has an outer diameter 25 that typically statically engages thecavity in cone 17. Seal 19 in this embodiment thus typically rotateswith cone 17. Seal 19 is preferably formed as a single piece member of asingle homogeneous polymer material, such as the following: nitrilebutadiene rubber (NBR); hydrogenated nitrile butadiene rubber (HNBR);carboxylated nitrile butadiene rubber (XNBR); or hydrogenatedcarboxylated nitrile butadiene rubber (XHNBR). The material of seal 19may also contain reinforcing materials, such as carbon black, silica orfibers.

Although a single homogenous material, at least one property of at leasta portion of seal 19 differs from the exterior surface of seal 19 to itsinterior. A property gradient 27 extends inward a selected distance fromthe exterior of seal 19. By way of example, property gradient 27 isshown in FIG. 4 extending inward from inner diameter region 23, howeverit could be alternately or also located on outer diameter region 25 orthe entire exterior surface. Property gradient 27 has at least oneproperty that changes from a first level, at the surface of seal 19, toa second level within the interior of seal 19. The property may behardness, elastic modulus, bulk modulus, toughness, abrasion resistance,friction coefficient, strength and other characteristics. For example,property gradient 27 is harder at the surface of inner diameter region23 and reduces in hardness in a direction toward the interior of seal19.

Property gradient 27 has a depth along an X-axis, which is a thicknessof seal 19 from inner diameter 23 to outer diameter 25, which isconsiderably less than about one-half the total distance from innerdiameter 23 to outer diameter 25. In the preferred embodiment, the depthof property gradient 27 in the X-direction is less than about 1/10^(th)the transverse cross-sectional diameter of seal 19 along the X-axis. Thedepth of property gradient 27 in the X-direction is greater than aboutone percent of the transverse cross-sectional diameter of seal 19 alongthe X-axis. For example, for a seal having a transverse cross-sectionaldiameter along the X-axis of 0.250″, the depth of property gradient 27is preferably only about 0.010-0.040″.

The Y-dimension 28 of property gradient 27 along the Y-axis, which isperpendicular to the X-axis, may be less than the total cross-sectionaldimension of seal 19 along the Y-axis. For a circular cross-section, asshown, the cross-sectional dimension of seal 19 in the above example is0.250″. The Y-dimension 28 of property gradient 27 in its natural state(FIG. 2) prior to installation should be sufficient to present theproperty gradient 27 of seal 19 to bearing pin 15 when installed. FIG. 3shows seal 19 installed under a typical operational squeeze along theX-axis. Squeezing seal 19 along the X-axis increases the surface contactbetween inner diameter 23 and bearing pin 15. The bulk of seal 19 willremain at a different property value than the property values inproperty gradient 27, as indicated in FIG. 4 for hardness. The surfacearea of property gradient 27 is less than the remaining portion of theexterior surface of seal 19 in this example.

In one embodiment, property gradient 27 has a hardness measured in termsof IRHD (International Rubber Hardness Degrees) utilizing amicro-hardness tester. The first level, at the exterior surface of seal19 is preferably at least about 10 percent greater than the second levelin property gradient 27, which is the average value within the interiorof seal 19. In one example, the IRHD is in the range from 85 to 90 atthe surface and 75 in the interior. These values correspond generally toShore A hardness values. Conventional measuring instruments formeasuring Shore A will not accurately resolve the difference in hardnesswithin property gradient 27.

Property gradient 27, if in rotary sliding contact, wears less due toits properties than if inner diameter region 23 were of the sameproperties as the interior of seal 19. One additional advantage ofretaining the majority of seal 19 at a lower compressive modulus thanthat in property gradient 27 is that the force developed by the sealwhen compressed along the X-axis to its compressed position is less thanif the seal were of a uniform higher stiffness. The lower compressivemodulus in the majority of seal 19 thus results in reduced contactstress and consequently friction at the seal dynamic interface.

Seal 19 could be of different shapes. For example, FIG. 5 shows a seal55 that in its natural configuration is elliptical. Seal 55 has agreater cross-sectional thickness along its X-dimension than itsY-dimension, creating a major axis along in the X-direction and a minoraxis in the Y-direction. Seal 55 has an inner diameter 61 and an outerdiameter 63. Property gradient 65 is shown located on the inner diameterregion 61 in this embodiment, but could be on the outer diameter region63 or the entire exterior surface.

There are a number of ways to achieve the desired property gradient 27.In one method, seal 19 (or seal 55) is molded conventionally to form arubber compound with generally uniform properties throughout. During theconventional molding process, which utilizes pressure and temperature,cross links are formed between the polymer chains. The cross linkingoccurring in the conventional process is self-limiting and utilizes across-linking agent, such as an organic peroxide. The cross-linkingstops when the cross-linking agent is consumed. Typically, a co-agent isemployed along with the cross-linking agent.

Subsequently, seal 19 is treated to create property gradient portion 27without changing the essential chemical composition of the material ofseal 19. In one technique, an electron beam process is employed togenerate property gradient portion 27. Seal 19 is placed in a chamberwithin an inert atmosphere, such as nitrogen. A focused electron beam isdirected through a window into the chamber, striking at least a portionof the exterior surface of seal 19. If seal gradient 27 is to be only ona fractional portion of seal 19, a shield will be employed to mask theelectron beam from the remaining portion. The depth of property gradient27 is controlled by the accelerating voltage of the electron beam. Theradiation dosage varies the properties within property gradient 27. Theradiation dosage is controlled by the time of exposure. In oneembodiment, the electron beam has an accelerating voltage in the rangefrom 70 kV to 250 kV. The radiation dose delivered by the electron beamis in the range from 10 kGy to 300 kGy. During the electron beamprocess, additional cross-linking of the polymer chains occurs to createproperty gradient 27.

In a second method, a radiation cross-link promoter (prorad) is utilizedto enhance the further cross-linking. The prorad is included into thepolymeric formulation prior to molding. The prorad survives the moldingprocess and may serve to facilitate the cross-linking of the elastomerduring the electron beam irradiation process. Suitable prorads arecommercially available for electron beam curing of polymers. Thefollowing lists the prorad by chemical name and in some instances, bytrademark and manufacturer:

-   diallyl maleate,-   triallyl cyanurate (TAC)-   triallyl isocyanurate (TAIC)-   n,n′-(m-phenylene bismaleimide (HVA-2)-   polyacrylates and polymethacrylates-   trifunctional acrylate, e.g. Saret SR.519-   trifunctional methacrylate, e.g. Saret SR-517-   pentacrythritol tetraacrylate, e.g. Saret SR-295-   dipentaerythritol pentaacrylate, e.g. Saret SR-399-   trimethylolpropane trimethacrylate, e.g. Saret SR-350-   liquid butadienes with 1,2-vinyl content 45% or higher (e.g. Ricon    100, Ricon 153, Ricon 154, Krasvl LB 2000, Krasvl LB 3000, Lithene    AH, Lithene AX)-   Methacrylated polybutadiene (e.g. Ricacryl 3100)-   Saret, Ricon and Ricacryl are trademarks of Sartomer Company, Inc.-   Krasvl LB is a trademark of Kancrik A.S.-   Lithene is a trademark of Synthomer Ltd.

The elastomeric formulation may contain any of the above prorads aloneor as a combination of two or more. The amount of prorad to be added tothe elastomeric formulation is about 2 to 20 phr (parts per hundredparts of rubber). In some cases, the co-agent used during theconventional molding process can also function as a prorad duringradiation cross-linking.

In some applications, the outer diameter region of a seal may be indynamic, sliding engagement while the inner diameter region is in staticengagement. In those cases, the property gradient may be only on theouter diameter region, if desired. In other cases, the inner and outerdiameter regions could be alternatively in sliding contact, in whichcase, both the inner diameter and the outer diameter regions would havea property gradient. Another embodiment would be for a property gradientto exist completely around the surface of the seal such that any sealsurface in sliding contact would have the property gradient.

Another embodiment would allow for the formation of a property gradientin non-axisymmetric areas of the seal. Such areas could compriseselected regions on a sector or multiple discontinuous regions orsectors of the seal.

In addition to seals for drill bits, seals for other subterraneanapplications are feasible, particularly for downhole well and miningtools. In addition to the materials for seal 19 mentioned above, othersuitable materials for seals for downhole well tools includefluorocarbon elastomers, perfluorocarbon elastomers, andfluorocarbon/propylene copolymer elastomers.

The invention has significant advantages. The method provides a propertygradient in desired areas without changing the chemical composition ofthe seal. The properties on the exterior provide better wear resistancefor dynamic engagement. The different properties in the interior or bulkportion of the seal avoid excessive force being generated due todeformation when installed.

While the invention has been described in only a few of its forms, itshould be apparent to those skilled in the art that it is not so limitedbut is susceptible to various changes without departing from the scopeof the invention. For example, the method of forming the propertygradient may be by gamma ray or x-ray processes.

1. A method of forming a subterranean drilling tool, comprising: (a)molding an annular seal from an elastomeric material of generallyuniform composition; (b) exposing the seal to an environment to form aproperty gradient on at least a portion of the seal defining a propertythat changes from a first level to a second level without essentiallychanging the composition of the elastomeric material; (c) forming a sealgland between a rotating and a static component; and (d) placing theseal in the seal gland.
 2. The method according to claim 1, wherein step(b) comprises irradiating said at least the portion of the seal.
 3. Themethod according to claim 1, wherein step (a) further comprisescross-linking the elastomeric material of the seal prior to step (b). 4.The method according to claim 3, wherein step (b) comprises utilizingradiation energy to further increase the increase a density of thecross-linking within the portion of the seal.
 5. The method according toclaim 1, wherein: step (a) further comprises including a radiationcross-linking promoter in the elastomeric material; and step (b)comprises directing an electron beam against the portion of the seal. 6.The method according to claim 1, wherein the property of step (b)comprises a selected one of the group consisting of hardness, elasticmodulus, and bulk modulus.
 7. The method according to claim 1, whereinstep (b) comprises controlling the exposure to the environment to createa depth for the property gradient that is less than about one-half of across-sectional thickness of the seal.
 8. The method according to claim1, wherein step (b) comprises controlling the exposure to theenvironment to create a depth for the property gradient that is at leastabout 10 percent of a cross-sectional thickness of the seal.
 9. Themethod according to claim 1, wherein step (b) comprises controlling theexposure to the environment to create a depth for the property gradientthat is at least about one percent of a cross-sectional thickness of theseal.
 10. The method according to claim 1, wherein the property of step(b) comprises an IRHD (International Rubber Hardness Degree) value, andthe second level differs from the first level by approximately at leastten percent.
 11. A method of forming a subterranean drilling tool,comprising: (a) molding an annular seal from a polymeric material ofgenerally uniform composition; (b) exposing at least a portion of theseal to a radiation source to form a property gradient on at least aportion of the seal; (c) forming a seal gland between a rotating and astatic component; and (d) placing the seal in the seal gland.
 12. Themethod according to claim 11, wherein the radiation source of step (b)comprises an electron beam.
 13. The method according to claim 12,wherein the electron beam has an accelerating voltage in the range from70 kV to 250 kV.
 14. The method according to claim 11, wherein step (b)comprises applying an electron beam dose in the range from 10 kGy to 300kGy.
 15. The method according to claim 11, wherein: step (a) furthercomprises including a radiation cross-linking promoter in the polymermaterial.
 16. The method according to claim 11, wherein: step (a)comprises including cross-linking co-agents in the polymer materialprior to molding the seal to cause an initial cross-linking of thepolymer material during the molding.
 17. The method according to claim16, wherein a portion of the cross-linking co-agents remain after themolding to enhance further cross-linking during step (b).
 18. A methodof forming a seal, comprising: (a) molding an annular member from anelastomeric material of generally uniform composition; and (b) exposingat least a portion of the member to an environment to form a propertygradient on said at least a portion of the seal defining a property thatchanges from a first level to a second level without essentiallychanging the composition of the elastomeric material.
 19. The methodaccording to claim 18, wherein step (b) comprises irradiating said atleast the portion of the member.
 20. The method according to claim 18,wherein step (a) further comprises cross-linking the elastomericmaterial of the member prior to step (b).